Heart lung interactions during mechanical ventilation.

PURPOSE OF REVIEW: To survey the recent medical literature examining studies of the hemodynamic effects of mechanical ventilation.\ud \ud RECENT FINDINGS: Ventilation-induced dynamic changes in arterial pulse pressure and stroke volume variation (PPV and SVV, respectively) identify volume responsiveness. The cause of PPV and SVV are due to intrathoracic pressure-induced variations in right atrial pressure changing intrathoracic blood volume over the ventilatory cycle. This explains why PPV and SVV are inaccurate with smaller tidal volumes used in acute lung injury, but remain useful in one-lung ventilation and prone positioning. Noninvasive measures of PPV and SVV using finger plethysmography and aortic root ultrasound or estimates of intrathoracic blood volume by thoracic impedance also predict volume responsiveness. Finally, the PPV-to-SVV ratio varies with vasomotor tone and can be used to identify vasopressor need in hypotensive patients. The clinical implications of these findings are starting to be realized in recommended management principles.\ud \ud SUMMARY: PPV and SVV predict volume responsiveness, but like all monitoring approaches, need to be understood within the framework of their physiologic determinations.\ud \u